ADC architecture for wireless applications

ABSTRACT

A simplified architecture is disclosed for use in wireless receiver applications, particularly for ADC conversion of received in-phase I and quadrature Q signals. Circuit area is substantially reduced by sharing a single ADC to convert both signals, switching the ADC input alternately between the i and q signals. In an embodiment, the ADC is clocked at an increased sample rate, and the digital output signals are aligned to compensate for the phase difference resulting from the implementation of a single ADC.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. Ser. No. 10/170,306filed Jun. 12, 2002. The disclosure of the above application isincorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention pertains to wireless communications and, morespecifically, to improved methods and apparatus for analog to digitalconversion in a voice and/or data communication receiver.

BACKGROUND OF THE INVENTION

Most wireless receiver circuits employ similar architectures. On thetransmitter side (not shown), digital user data (which may be encoded orsampled voice data) is input to a bit-symbol encoder, which may beimplemented as a dedicated circuit or a combination of hardware andsoftware (such as a DSP). In any event, the bit-symbol encoder typicallysplits the data into two half-rate streams. These streams are modulatedonto a pair of orthogonal carriers, called I and Q for in-phase andquadrature-phase carriers respectively, although other modulationschemes are known. This is a 2-D (or two-dimensional) transmissionscheme. The I and Q signals are combined for transmission over an RFchannel as is well known in the art.

Referring now to FIG. 1, a typical wireless receiver circuit 10 has anRF antenna 12 to receive an incident RF signal. The RF signal from theantenna may be subjected to a band pre-select filter (not shown), andtypically is input to a low-noise amplifier 14 (LNA). The filtered,amplified RF signal is input to a first mixer 16, to bring the RF signaldown to a selected intermediate frequency (IF). Mixer 16 is driven by afirst local oscillator signal LO1 applying techniques that are wellknown in the radio prior art.

An IF amplifier 18 and/or low pass filter (not shown) further conditionsthe signal, which is then split into two paths. The IF signal is inputto a second mixer 20 and also input to a third mixer 24. The secondmixer 20 is driven by a second local oscillator signal LO2, arranged torecover or demodulate the in-phase baseband analog signal “i”. The localoscillator signal LO2 is also input to a 90-degree delay or phaseshifter 22, the output of which drives the third mixer 24. The IF signalis input to the third mixer as illustrated, so that the third mixer 24demodulates the quadrature-phase orthogonal baseband analog signal “q”.In this description, we use lower case “i” and “q” to refer to theanalog signals corresponding to the I and Q digital data streams,respectively.

Next the in-phase analog signal “i” is input to a low pass filter 30,and then to a buffer 32. The buffered signal is input to ananalog-to-digital converter circuit (“ADC”) 34 for conversion intodigital data. This stage recovers, at node 35, one of the pair ofhalf-rate data streams originally encoded in the transmitter.

Similarly, the quadrature phase signal “q” recovered by the third mixer24 is filtered in LPF 40 and buffer 42. The resulting analog signal isinput to a second analog to digital converter circuit 44 (ADC) forconversion into digital data. This stage recovers, at node 45, the otherone of the pair of half-rate data streams originally encoded in thetransmitter. Both ADC's are driven by a common receive clock signal 48(“RX clock”), which may have a frequency of, for example, 44 MHz. Thetwo ADCs are thus synchronized; they operated on the i and q signals inparallel. The recovered digital data streams are further processed in apair of FIR low-pass filters LPF 36, 46 respectively. The two digitalstreams are then further processed by other functional blocks such as acomplex equalizer and correlators as is known in the art.

Various ADC circuits are known that could be used in this architecture.They include the conventional feedback-type ADC, which is small butslow. U.S. Pat. No. 5,229,771 to Hanlon describes an integrating type ofADC circuit in which switches, like multiplexers, are used to selectvarious reference voltages Vref+, Vref− and trip voltages Vtrip(n) etc.for use in the integration stage of the ADC; but that system stillconverts only a single analog input signal Vin during a cycle.

A faster approach is realized in a parallel or “flash” ADC. A flash ADCincorporates a plurality of comparators arranged in parallel, so as toform the N-bit digital output signal in a single cycle. U.S. Pat. No.6,169,504 describes the use of interleaved sampling for fasterconversion in the context of a parallel series of comparators. Improvedaccuracy can be obtained by using pipelined ADC circuits. These too areknown, as described for example in U.S. Pat. No. 6,166,675.

One significant drawback of the receiver architecture summarized aboveis related to the pair of ADC circuits. Mismatches in the internalcomparators of the ADCs must be carefully controlled to minimize errors.Especially in pipelined converter designs, offset, gain errors and otherimperfections in the ADC circuits add to their complexity. See U.S. Pat.No. 5,594,445. Consequently, high performance ADC circuits are expensiveboth in terms of chip area and power consumption. These factors are ofparamount importance in integrated circuit designs for portableapplications such as cell phones, pagers and other wirelesscommunication equipment.

Accordingly, it would be advantageous to reduce the amount of chip areathat must be dedicated to ADC circuits in a receiver. It would also beadvantageous to reduce the number of ADC circuits required in areceiver, without sacrificing speed or accuracy of received datarecovery.

SUMMARY OF THE INVENTION

One aspect of the present invention is an improved baseband circuit foruse in connection with an RF receiver for wireless communication ofdata. The new baseband circuit provides a substantial reduction incircuit area and power consumption by reducing the number of ADCcircuits necessary for conversion of analog data streams. According toone embodiment, the receiver provides “i” and “q” orthogonal analogsignals from corresponding demodulators. These two signals are convertedto digital data in a single, shared ADC circuit, thereby substantiallyreducing circuit size and power.

Another aspect of the invention provides a method for concurrentlyconverting a pair of orthogonal signals into corresponding digitalsignals (vectors) in a baseband processor, using only a single ADCcircuit. The method in one embodiment calls for sampling a first one ofthe analog signals in the ADC circuit during a first time interval toprovide a first digital signal, and sampling the other one of the pairof analog signals in the same ADC circuit during a second time intervalnon-overlapping the first time interval to provide a second digitalsignal. The two digital output signals are then aligned so as tocompensate for a phase difference introduced by the alternating samplingsteps.

The alignment can be accomplished by various digital processingtechniques, essentially by delaying the two signals by differentintervals to bring them into alignment. This is preferably done usingdigital delay and LPF interpolation circuits.

Additional aspects and advantages of this invention will be apparentfrom the following detailed description of embodiments thereof, whichproceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a known receiver circuit.

FIG. 2 is a simplified schematic diagram of a baseband processingcircuit according to a first embodiment of the invention, for use in areceiver circuit of the general type shown in FIG. 1.

FIG. 3 illustrates a baseband processing circuit according to a secondembodiment of the invention.

FIG. 4 illustrates a method of sampling a pair of waveforms using only asingle ADC circuit consistent with the present invention.

FIG. 5 is a flow diagram illustrating a baseband processing methodconsistent with the present invention.

FIG. 6 is a simplified block diagram of a transceiver incorporating asingle ADC architecture in accordance with the present invention.

FIG. 7 is a simplified schematic diagram of one example of phasecorrection circuitry in the baseband processor, employing a linearinterpolator characterized by (1+D)/2.

FIG. 8 is a simplified schematic diagram of another example of phasecorrection circuitry in the baseband processor, employing aninterpolator characterized by (−1+3D+3D²−D³)/4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is directed to improvements in receiver circuitsgenerally of the type illustrated in FIG. 1. It is not limited to 2-Dmodulation, however, but could be used in any multi-dimensionalmodulation RF communications application. For simplicity, we describebelow particular embodiments of the invention, directed to a 2-D (I,Q)modulation application.

As described above with reference to FIG. 1, a typical RF or wirelesscommunications receiver includes RF input, filtering, down-mixing etc.all generally designed to recover the I and Q analog signals. We willrefer to such circuitry as the RF portion of the receiver, although itincludes IF operations as well. Using this working definition, the RFportion provides the baseband analog signals i, q at its output. The RFportion 56 of the receiver of FIG. 1 is the circuitry generally to theleft of the dashed line.

The circuitry 60 on the right side of FIG. 1, including in particularthe pair of ADCs, processes the analog baseband signals to recover theoriginal digital data. We will refer to this type of circuitry,generally to the right of dashed line in the illustrative receivercircuitry of FIG. 1, as a baseband processing circuit, or simply“baseband processor”. Referring now to FIG. 2, one embodiment of animproved baseband processor in accordance with the present invention isshown. This type of improved baseband processor can be used incombination with various receiver circuits of the general typeillustrated in FIG. 1, namely any RF receiver that down-mixes ordemodulates received signals so as to recover at least two analogsignals that encode digital data symbols. The present invention is notlimited to any particular symbol coding.

The task of the new baseband processor of FIG. 2 is essentially tore-create the encoded digital data, through analog to digitalconversion, while overcoming the shortcomings of prior art such as thosementioned above. Specifically, the new circuit 200 reduces circuit areasignificantly by reducing the number of ADCs from two to one. In FIG. 2,the “i” analog signal (corresponding to the output of filter 30 inFIG. 1) is input to a first buffer 202 to provide low output impedanceto drive the subsequent ADC. Similarly, the “q” analog signal(corresponding to the output of filter 40 in FIG. 1) is input to asecond buffer 204. The outputs of buffers 202,204 are input torespective signal inputs to a first multiplexer 206, a 2:1 multiplexer.

The signal output of multiplexer 206 is input to an ADC circuit 210,which in the present embodiment comprises a flash ADC with 6-bitaccuracy although the particular choice of ADC depth is not criticalwith respect to the present invention. The digital output vector stream,after delayed by several clock cycles due to internal conversion delay,is input to a demultiplexer circuit 214. Details of these individualcircuit components, such as the multiplexer and demultiplexer circuits,are omitted as they are well known in the art.

The demultiplexer 214 alternately provides output vector streams at node216 and node 218. A delay circuit 220 receives digital data from thefirst output 216 of the demultiplexer, while an interpolator circuit 222receives digital data from the second output 218 of the demultiplexer214. (The term “while” in the preceding sentence is not intendedliterally to imply simultaneity; rather, in present embodiment, data isoutput from the demultiplexer on alternate phases or clock cycles asmore fully explained below.) Finally, the delay circuit 220 and theinterpolator 222 provide the recovered I and Q digital data streams,respectively.

Multiplexer 206 and demultiplexer 214 each have a control input 208, 209respectively. These inputs are driven by a “multiplexer control” signal,which is a periodic clock signal having the same frequency as the clocksignal that drives the ADC at clock input 212. The clock signalsgenerally are provided by on-chip PLL circuits or the like (not shown).In the present embodiment, the ADC clock signal 212 (and the multiplexercontrol clock signals 208, 209) has a frequency that is significantlyhigher than the clock rates of prior art baseband circuits, such as theRX clock signal of FIG. 1. For example, the known circuit of FIG. 1 mayemploy a 44 MHz receive clock to drive both ADC's 34 and 44, while oneembodiment of the circuit of FIG. 2 uses an 88 MHz signal to clock thesingle ADC. We will refer to the signal at 212 as the “RX2” clock.Because both the I and Q streams share a single, common ADC circuit, thenet result is essentially the same ADC bandwidth as the prior art, whilechip area is substantially reduced. It is estimated that the additionaloverhead of circuits that employ the present invention will amount toonly about 10% of the saved chip area that would otherwise be necessaryto accommodate a second ADC, as in circuits of the type illustrated inFIG. 1.

In operation of the circuit of FIG. 2, the multiplexer control signal208 first selects the “i” signal (from buffer 202) and the multiplexer206 conveys it to the ADC 210 analog input. The ADC samples the inputsignal during a first cycle. The ADC is driven by the RX2 clock which issynchronized with, and at twice the frequency of the multiplexer controlsignal. During the next clock cycle, the multiplexer 206 selects the “q”signal (from buffer 204) and conveys it to the ADC signal input. On thenext clock cycle, the multiplexer switches back to the first (“i”) inputsignal, and so on, so that the ADC 210 is alternately sampling the twoinput signals in “ping-pong” fashion. The multiplexer control signal issimply RX2 divided by 2.

Samples from both analog input signals are thus converted to digitalform in one ADC 210. (The resulting digital data appears at the ADCoutput some number of clock cycles after sampling the input, dependingon the depth/latency of the ADC pipeline.) The multiplexer controlsignal or its equivalent also controls the demultiplexer 214 at itscontrol input 209. In other words, the multiplexer and demultiplexercontrol signals 208, 209 are coordinated such that the demultiplexer 214steers the ADC digital output to the first output node 216 during aclock period corresponding to input of the “i” signal to the ADC. Thusthe “i” data stream digital data is output to the delay element 220.Conversely, the demultiplexer 214 steers the ADC digital output to thesecond output node 218 during alternate clock periods corresponding toinput of the “q” signal to the ADC as described above. Thus the “q” datastream digital data is output to the interpolator element 222. Theseoperations continue responsive to the RX2 clock and control signalsdescribed above.

In the embodiment shown in FIG. 2, the ADC 210 is driven at a greatersample rate (clock rate) than in the conventional, two-ADC type ofcircuit. Because the ADC is shared between the two input signals, aclock frequency of twice that of the circuit of FIG. 1 will yieldessentially the same bandwidth as FIG. 1. The particular clock speed fora given application is a matter of design choice, trading off speed vs.power and other considerations. The clock speed is not critical to theinvention, as long as it can accommodate analog to digital conversationof the i and q at the desired I and Q sampling rates. The circuits andoperations described above not only reduce chip area, but the use of anincreased ADC sample rate (clock speed) also improves latency in theADC.

The baseband processor circuits and methodology described thus far withregard to FIG. 2 result in a phase difference between the recovered Iand Q data streams. FIG. 4 illustrates this effect. In FIG. 4, signals“i”, “q” and ADC Clock are illustrated in a simplified, idealized timingdiagram. Assume the ADC samples its input beginning at the rising edge410 of the clock signal, and assume the i signal is sampled first, atpoint 412 on the i waveform. On the next clock cycle, rising edge 414,the input multiplexer is switched, and the ADC samples the q waveform atpoint 416. On the next clock, the multiplexer is switched again, and theADC samples the i waveform at point 418, and so on. The reader canobserve that the q sampling lags behind the i sampling; in other words aphase difference is introduced. That phase difference between therecovered I and Q streams can be corrected in various ways; someexamples follow. This embodiment implements the phase correction on thedigital data (I and Q), although it alternatively could be done in theanalog realm.

Referring again to FIG. 2, the I digital signal is delayed by one-halfcycle in a delay element 220. The Q signal is input to an interpolator(low-pass filter) 222. The interpolator is designed to realize (2k+1)/2cycles of delay, in other words 0.5 or 1.5 or 2.5 cycles, etc. This canbe done with a simple, symmetric 4-tap LPF with constant coefficients asis known. For example, the LPF can be described by the transfer function(−1+3D+3D²−D³)/4. The delay element 220 and the LPF 222 bring the I andQ signals into phase alignment for subsequent decoding. In general,after the ADC conversion, one signal is delayed and the other signalfiltered by an LPF interpolator to compensate for the phase differencearising from the implementation of a single ADC.

FIG. 7 is a simplified schematic diagram showing one example of phasecorrection circuitry in the baseband processor in greater detail. Thiscircuit implements a linear interpolator characterized by (1+D)/2. Inthis figure, as well as FIG. 8, CK is a half-rate clock signal in phasewith RX2. FIG. 8 is a simplified schematic diagram of another example ofphase correction circuitry in the baseband processor; this circuitemploying an interpolator characterized by (−1+3D+3D²−D³)/4.

FIG. 3 illustrates an alternative embodiment of the baseband processor.In FIG. 3, the baseband processor is similar to the circuit of FIG. 2,except as follows. The i and q analog signals are input to a multiplexer302, without the use of buffers (202, 204 in FIG. 2). Multiplexer 302still serves to steer these signals, alternately, to the ADC as before.In FIG. 3, however, a buffer 306 is introduced between the multiplexer302 and the ADC. Various arrangements of buffers can be used as long asthey provide adequate drive into the ADC.

FIG. 5 is a flow diagram illustrating a baseband processing methodaccording to the present invention. The methodology can be implementedin a variety of ways, using one or more information processors such asgeneral purpose or special purpose processors, and/or programmable logicas is well known in the art. Referring to FIG. 5, the inventive methodcan be summarized as directed to concurrently converting a plurality ofanalog signals having a predetermined phase relationship intocorresponding digital signals, while using only a single ADC circuit.The initial step 502 calls for providing a single ADC circuit, asexplained above. Next, during a first time interval 504, sampling 506 afirst one of the analog signals in the ADC circuit to provide a firstdigital signal 508. Further, during a second time interval 510non-overlapping the first time interval, the method calls for sampling512 a second one of the analog signals in the ADC circuit to provide asecond digital signal 514.

In a phase quadrature application, there are only two analog signals todecode, I and Q. Other schemes may involve more signals. In the lattercase, the method further calls for sampling 520 each remaining analogsignal, if any, in the same ADC circuit, so as to provide zero or moreadditional digital signals 522. These additional sampling steps would becarried out during allocated time slots 530 non-overlapping the previoustime intervals.

Next the process calls for aligning the first, second (and anyadditional digital signals), reference 540, so as to restore theoriginal phase relationship among the corresponding analog signals.These sampling and aligning steps are repeated substantiallycontinuously in response to a periodic clock signal as explained above.

FIG. 6 is a simplified block diagram of an example wireless transceiver600 incorporating the ADC circuit 200 shown in FIG. 2 as part of thebaseband processor 60 within receiver 680. As shown in FIG. 6, in thisembodiment the digitized I and Q signals recovered by the ADC circuit200 of the baseband processor are fed to the symbol decoder 650 torecover the data of interest. Though not required, it should be realizedthat the I and Q signals typical present data in symbol-encoded form,using orthogonal class modulation techniques such ascomplementary-code-keying (“CCK”) or OFDM modulation specified in IEEEStd. 802.11b (1999) and 802.11a (1999) HIGH-RATE PHY extensions toIEEE/ANSI Std. 802.11 (1999), all of which incorporated herein fully byreference, in order to achieve high-speed transmission yet retaincommercially practicable recovery rates.

After symbol-decoding by the symbol decoder 650, the recovered data issent to a data interface such as MAC layer interface 610 for furtheroperation. As shown in FIG. 3, the transceiver 600 also includes aconventional transmitter 660 to carry out encoding and transmission ofoutbound data from the MAC interface 610. Though not required as will beappreciated by those of ordinary skill, the embodiment of FIG. 6 depictsthe transmitter 660 and receiver 690 sharing a common RF antenna 692.

Though not shown in FIG. 6, the transceiver 600 may form an operationalpart of a network interface apparatus such as a PC card or networkinterface card capable of interfacing with the CPU or informationprocessor of an information processing apparatus such as a desktop orlaptop computer, and may be integrated within and constitute a part ofsuch information processing apparatus. This network interface apparatusmay alternatively form an operational component of a wirelesscommunications access point such as a base station as will beappreciated by these ordinarily skilled in the art.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments of thisinvention without departing from the underlying principles thereof. Thescope of the present invention should, therefore, be determined only bythe following claims.

1. A method comprising: providing an analog to digital converter (ADC);during a first time interval, sampling a first one of a pair of analogsignals in the ADC to provide a first digital signal; during a secondtime interval non-overlapping the first time interval, sampling theother one of the pair of analog signals in the ADC to provide a seconddigital signal, wherein the pair of analog signals have a predeterminedphase relationship; and aligning the first and second digital signals tocompensate for phase difference introduced by the sampling during thefirst and second time intervals, wherein the aligning comprises:delaying one of the first and second digital signals; and interpolatingthe other one of the first and second digital signals to compensate forthe phase difference introduced by the sampling during the first andsecond time intervals, wherein interpolating the other one of the firstand second digital signals realizes a delay of (2k+1)/2 clock cycles,where k is an integer.